In an IPS liquid crystal display device configured to operate in an IPS mode, liquid crystal molecules have, in a non-drive state, a homogeneous orientation approximately parallel to a plane of each of a pair of opposed substrates, so that light can pass through a liquid crystal layer almost without a change in polarization plane thereof. Therefore, in the non-drive state, almost perfect black display can be achieved by disposing two polarizing plates, respectively, on upper and lower sides with respect to the substrates. In the IPS mode, almost perfect black display can be achieved when viewing a liquid crystal display panel in a direction normal thereto. However, when viewing the liquid crystal display panel from a direction deviating from the normal direction, light leakage inevitable due to properties of a polarizing plate occurs in a direction deviating from a direction of an optical axis of the polarizing plates disposed on upper and lower sides of a liquid crystal cell, thereby leading to a problem that a viewing angle becomes narrow.
With a view to solving this problem, for example, in JP 3687854B (Patent Document 1), parameters (such as refractive index, retardation and thickness) of an optical film obtained by laminating a polarizing plate and a retardation film such that an absorption axis of the polarizing plate becomes orthogonal or parallel to a slow axis of the retardation value, are set to suitable values for an IPS liquid crystal display panel. In one example, a 20 μm-thick polarizing plate having two 80 μm-thick transparent protective films laminated, respectively, to opposite surfaces thereof using an adhesive, and a 45 μm-thick retardation film, are used.
In JP 4938632B (Patent Document 2), there is described an IPS liquid crystal display device in which a polarizer having a thickness of 10 μm to 50 μm is used. In JP 4804588B (Patent Document 3), there is described a structure in which a retardation film is bonded to a viewing-side surface of an IPS liquid crystal display panel through a pressure-sensitive adhesive layer, and a polarizer having a thickness of 10 μm or less is bonded to the retardation film. In JP 4757347B (Patent Document 4), there is described an optical film serving as a retardation film and having a thickness of 20 μm or less.
The IPS liquid crystal display panel used with optical films such as a polarizer and a retardation film is not only employed in a relatively large-sized screen for liquid crystal televisions, liquid crystal displays and the like, but also recently applied to a screen for mobile small-sized electronic devices, such as a smartphone and a tablet PC. Recent years, along with a reduction in thickness of such mobile electronic devices, an IPS liquid crystal display panel has been produced to become thinner.